This invention relates to the measurement of optical fiber characteristic during payout of the optical fiber from a support, and, more particularly, to the measurement of the electromagnetic field of the optical fiber during payout.
Optical fibers are strands of glass fiber processed so that light transmitted therethrough is subject to total internal reflection. A large fraction of the incident intensity of light directed into the fiber is received at the other end of the fiber, even though the fiber may be hundreds of meters long. Optical fibers have shown great promise in communications applications, because a high density of information may be carried along the fiber and because the quality of the signal is less subject to external interferences of various types than are electrical signals carried on metallic wires. Moreover, the glass fibers are light in weight and made from a highly plentiful substance, silicon dioxide.
Glass fibers are fabricated by preparing a preform of glasses of two different optical indices of refraction, one inside the other, and processing the preform to a fiber. The optical fiber is coated with a polymer layer termed a buffer to protect the glass from scratching or other damage. As an example of the dimensions, in one configuration the diameter of the glass optical fiber is about 125 micrometers, and the diameter of the fiber plus the polymer buffer is about 250 micrometers (approximately 0.010 inches).
For such very fine fibers, the handling of the optical fiber to avoid damage that might reduce its light transmission properties becomes an important consideration. The fibers are typically wound onto a cylindrical or truncated conical bobbin with many turns adjacent to each other in a side-by-side fashion. After one layer is complete, another layer of fiber is laid on top of the first layer, and so on. An adhesive is applied to the layers to hold them in place on the canister, and to control the payout of the optical fiber to avoid snarls and breaks that might result from payout of multiple turns at once. The final assembly of the bobbin and the wound layers of fiber is termed a canister, and the mass of wound fiber is termed the fiber pack. When the optical fiber is later to be used, the fiber is paid out from the canister in a direction parallel to the axis of the cylinder.
One basic problem during the payout of the optical fiber from the canister is the measurement of the amount of optical that has been paid out. The optical fiber may be paid out at rates as high as several hundred meters per second. Mechanical gauges are not practical because contact of anything to the fiber may damage it. Various types of optical sensors have been tried. These gauges utilize light sources and detectors that detect the optical fiber operating on the principle of interruption of the light beam, reflection, or refraction. These gauges are often bulky, require considerable power, are sensitive to stray light and reflections, and may be inaccurate if gauge placement and calibration are not extremely precise.
There exists a need for an approach for accurately measuring an optical fiber as it is payed out from a canister at rates as high as several hundred meters per second. The apparatus used to do the measurement should ideally consume little power, be accurate, and be compact. The present invention fulfills this need, and further provides related advantages.